1. Field of the Invention
The present invention relates generally to a method and apparatus for transmitting at least one incoming digital signal in a data channel having a constant clock rate, in the case of which the data rates are matched by positive-zero-negative stuffing in that, on reaching an upper decision threshold and a lower decision threshold of the phase separation, additional stuffing processes are produced, beyond the required stuffing processes.
2. Description of the Related Art
The positive, the negative and the positive-zero-negative stuffing techniques are described in Section 1.4, pages 8 to 12 and 15 to 20, May 1983 issue and pages 13 to 14, May 1984 issue, of the loose-leaf document "Digitale Ubertragungstechnik" [Digital transmission technology], Kahl, R. v. Becker's Press, G. Schenck, Heidelberg.
In order to be able to transmit transmitted data signals in a data channel having a constant bit rate, the transmission bit rate must be matched to the bit rate of the digital signal. For this purpose, the transmitted data signal contains stuffing points at periodic intervals, which stuffing points contain bits of the digital signal or dummy bits, as required.
If the stuffing point is planned to contain data bits of the digital signal and dummy bits to the extent by which the bit rate of the digital signal lags behind that of the transmission bit rate reserved, the insertion of the dummy bits into the stuffing points is designated positive stuffing.
If the stuffing point is planned to contain dummy bits and data bits of the digital signal to the extent by which the bit rate of the digital signal exceeds that of the transmission bit rate reserved, the insertion of the data bits into the stuffing points is designated negative stuffing.
Stuffing methods which use both types of stuffing are designated positive-zero-negative stuffing methods.
These methods are used even when multiplex signals on a lower hierarchical level are to be combined to form a common multiplex signal on a higher hierarchical level.
Each stuffing process having a bit produces a phase change of the outgoing (transmitted) digital signal by one UI (unit interval). Positive stuffing causes the phase to lag by one UI with respect to the incoming digital signal; negative stuffing causes a phase lead of one UI. The phase changes are smoothed by means of a phase-locked loop PLL at the destination (for example during recovery of the transmitted signal from the multiplex signal). On the basis of its dimensioning, the phase-locked loop acts like a low-pass filter having a cutoff frequency f.sub.0. The time constant of the low-pass filter is T=1/(2*.pi.*f.sub.0). The amplitude of the phase changes (jitter) caused by stuffing is greatly reduced by the phase-locked loop if the time interval between the phase changes is very small in comparison with the time constant T, that is to say if the frequency with which the stuffing processes occur is very high in comparison with the cutoff frequency f.sub.0.
In the case of the positive-zero-negative stuffing method, the time intervals between the phase changes can be large in comparison with the time constant T. In this case, the amplitudes of the phase changes are not damped much by the phase-locked loop and the values of the jitter are virtually one UI.
From the publication "Jitter Analysis of Asynchronous Payload Mappings", T1X1.4/86-447, Contribution to T1 Standards Project-T1X1.4, 12.11.86, pages 9 and 10, "threshold modulation" is furthermore known, in the case of which thresholds of the phase difference between an incoming digital signal at the transmission end and an outgoing digital signal at the transmission end are varied, so that additional stuffing processes are produced. In this case, the jitter assumes a higher frequency which can be filtered more easily.
The European Patent Specification 0 248 551 A2 describes a method for synchronization of asynchronous data (stuffing method) which leads to an increase in the stuffing frequency and hence in the jitter frequency. The use of stepped characteristics for the threshold values for initiating the additional stuffing processes is also known from this document. In the case of this method, the superfluous stuffing processes are disadvantageous when the (synchronous) signal which is to be transmitted has the desired phase. The arrangement is likewise dependent on considerable circuit complexity.
The European Patent Application 0,192,492 likewise describes a method for increasing the stuffing frequency in the case of a positive-zero-negative stuffing method.